Metallacyclosilanes of Calcium, Yttrium, and Iron

Utilizing a choice of α,ω-oligosilanylene diides, it is possible to synthesize a number of heterocyclosilanes with heteroelements of calcium, yttrium, and iron by metathesis reactions with respective metal halides CaI2, YCl3, and FeBr2. 29Si NMR spectroscopic analysis of the calcacyclosilanes suggests that these compounds retain a strong oligosilanylene dianion character, which is more pronounced than in the analogous magnesacyclosilanes. As the electronegativity of calcium lies between potassium and magnesium, silyl calcium reagents should be considered as building blocks with an attractive reactivity profile. Reaction of a 1,4-oligosilanylene diide with YCl3 gave the five-membered yttracyclosilane as an ate-complex with two chlorides still attached to the yttrium atom. Reaction of the obtained compound with two equivalents of NaCp led to another five-membered yttracyclosilane ate-complex with an yttracene fragment. When using a dianionic oligosilanylene ligand containing a siloxane unit, the siloxane oxygen acted as an additional coordination site for Ca and Y. When the same ligand was used to prepare a cyclic 1-ferra-4-oxatetrasilacyclohexane, an analogous transannular interaction between the iron and oxygen atoms is missing.


■ INTRODUCTION
For a long time, the synthesis of heterocyclosilanes was either restricted to insertion reactions into strained cyclosilanes or to reactions of α,ω-oligosilanylene dihalides with nucleophilic heteroatoms. Preparation of metallacyclosilanes with nonnucleophilic metals, however, was more challenging. The development of a convenient synthetic protocol for α,ωoligosilanylene dianions 1 eventually permitted simple access to this class of compounds, where frequently the interaction between the metal and the neighboring silicon atoms is rather polar or at least weaker than typical silicon main-group bonds.
Silyl Calcium Compounds. One type of compounds with a weak silicon metal interaction are magnesacyclosilanes, which are easily prepared from dipotassium α,ω-oligosilanylene diide compounds by reaction with an equimolar amount of MgBr 2 · Et 2 O. 2−6 A major reason for our interest in these compounds was to moderate the reactivity of the associated silanides compared to that of dipotassium compounds. Magnesium silanides can be regarded as sila-Grignard-type reagents. Indeed, we could utilize these compounds in a number of reactions where potassium silanides were either too reducing, too basic, or too reactive in general. 7 Considering organomagnesium (Grignard) compounds, an obvious question is whether instead of magnesium calcium might be used as an alternative metal. A short answer to this question is, "In principle, yes, but these compounds are neither easily prepared as nor can be handled with the same ease as organomagnesium compounds." Nevertheless, in recent years, the chemistry of organocalcium compounds has been intensively investigated and several synthetically useful methods for their preparation have been developed. The direct (Grignard analogous) synthesis requires activated calcium and preferably organoiodides as starting materials. Other methods include the deprotonation of C−H acidic compounds with calcium amides and transmetallation/metalexchange reactions. 8−11 The synthesis of alkaline-earth derivatives of heavier congeners of carbanions, namely, silanides, is associated with similar problems. Although silyl magnesium compounds have gained wide acceptance over the last few years, they are typically not obtained from direct reactions of silyl halides with elemental magnesium 12 but rather by metathesis of lithium or potassium silanides with magnesium bromide. 13−18 The same is essentially true for silyl calcium compounds, the number of reported examples still being limited to very few (Chart 1). Remarkably enough, the very first report of calcium silanides by Mochida and Manishi is the only approach that utilizes the direct Grignard-type reaction of insertion of elemental calcium into a Si−Cl bond. 19 For this purpose, calcium metal vapor was used. Four different silyl calcium compounds (I) (Chart 1) were prepared by this method, and based on derivatization reactions with a number of different electrophiles the yields of formation were rather moderate. 19 All other approaches to calcium silanides reported so far have employed metathesis reactions of lithium or potassium silanides with CaI 2 . Teng and Ruhlandt-Senge demonstrated the principal feasibility of this by reaction of (Me 3 Si) 3 SiK with CaI 2 to [(Me 3 Si) 3 Si] 2 Ca (II) (Chart 1). 20 A report by the Sadow group on the formation of [(Me 2 HSi) 3 Si] 2 Ca (III) (Chart 1) was along the same lines. 21 Subsequently, Sekiguchi and co-workers showed that reaction of the dipotassium tetrasilacyclobutadiene dianion K 2 [( t Bu 2 MeSi) 4 Si 4 ] with CaI 2 led to the formation of an interesting building block calcium tetrakis(di-tertbutylmethylsilyl)tetrasilabicyclo[1.1.0]butane-2,4-diide (IV) (Chart 1). 22 More recently, Okuda and co-workers prepared (Ph 3 Si) 2 Ca (V) from Ph 3 SiK, 23 and Mills and Liddle reported the synthesis of THF adducts of ( t Bu 3 Si) 2 Ca and (tBu 2 MeSi) 2 Ca (VI) from the respective sodium silanides (Chart 1). 24 Silyl Yttrium Compounds. The origin of the organic chemistry of rare-earth metals dates back to the seminal work of Wilkinson and co-workers on metallocenes in the 1950s. 25 Though with a slow start, this area is nowadays an intensively investigated and flourishing field. 26 Despite a large number of known organic rare-earth metal compounds (containing metal−carbon bonds), examples of silylated rare-earth complexes are still scarce. 27 30 (X) (Cp′ = C 5 H 4 Me) was formed in the reaction of the Y(II) compound K[Cp′ 3 Y] with PhSiH 3 . Most recently, our group reported two disilylated yttrium atecomplexes XI and XII by reaction of a siloxane-containing oligosilanylene diide with YCl 3 and subsequently with CpNa. 31 Silyl Iron Compounds. While a considerable number of silyl iron complexes is known, examples with two silyl ligands are not so common, and the tris(trimethylsilyl)silyl group is the only oligosilanyl ligand for which disilylated iron complexes are known. Tilley and co-workers reported the reaction of (Me 3 Si) 3 3 Si] 2 Fe} (XIV) can be obtained directly from the reaction of (Me 3 Si) 3 SiK with FeBr 2 in THF, and they further reported its conversion to the dipyridine complex (Chart 3). 34 With the availability of several oligosilanylene diides, we decided to study the possibility of formation of ferracyclosilanes.
■ RESULTS AND DISCUSSION Silyl Calcium Compounds. Dipotassium oligosilanylene-1,4-diide 1 35,36 is arguably the compound that we used most frequently for the preparation of five-membered heterocyclosilanes. In particular, the compound was reacted with MgBr 2 · Et 2 O to obtain the respective magnesatetrasilacyclopentane, 4 which may be considered as a reference compound for comparison with other metallacyclosilanes with strong silanide character. Here we treated the dianionic compound 1 with CaI 2 to obtain the expected calcatetrasilacyclopentane compound 2 with a decent yield of 81% (Scheme 1).
To assess the reactivity of compound 2, it is convenient to estimate the silanide character by means of 29 Si NMR spectroscopic analysis. Typically, the silanide character correlates well with the up-field shift of the anionic silicon atom. The chemical shifts of the attached SiMe 3 groups are also indicative and compared to a neutral compound, typically a down-field shift of the SiMe 3 signal can be observed. The 29 Si NMR spectral properties of the dipotassium oligosilanylene-1,4-diide compound 1 are  Table 1). 37 In the context of these numbers, the 29 Si NMR spectrum of 2 [−5.1 (SiMe 3 ), −28.4 (SiMe 2 ), and −188.0 (SiCa) ppm] suggests the compound to be very ionic with the chemical shift of the metalated silicon atoms being very close to that of the starting material (Table 1). While this value is almost 16 ppm up-field-shifted compared to the bis[tris-(trimethylsilyl)silyl]calcium compound II reported by Teng and Ruhlandt-Senge (−172.3 ppm), 20 a similar but less pronounced behavior was observed for the comparison of the respective magnesacyclopentasilane (−176.6 ppm) 4 and the bis[tris(trimethylsilyl)silyl]magnesium compound (−171.9 ppm). 4,38 Single-crystal XRD analysis of 2 ( Figure 1) shows the compound to be a regular calcatetrasilacyclopentane with the calcium being further coordinated by two dimethoxyethane (DME) molecules. The five-membered ring is almost flat with all ring atoms except for one of the SiMe 2 units being co-Scheme 1. Reaction of Oligosilanylene-1,4-diide 1 with Calcium Diiodide to Calcacyclopentasilane 2 Table 1. 29 Si NMR Data of 1-Metalla-2,2,5,5-tetrakis(trimethylsilyl)-3,3,4,4-tetramethylcyclopentasilanes a The DME content of 2 in this case was determined by 1 H NMR spectroscopy as 1.5. This is different from the number of 2 DME molecules observed in the XRD analysis.  In order to extend this chemistry, we repeated the reaction of CaI 2 with the siloxane-containing oligosilanylene-1,5-diide 3 41 to obtain the disilanyl calcium compound 4 (Scheme 2).
Initially, we developed dianion 3 to obtain bidentate silyl ligands with an additional Lewis basic site. 41 It was used for the synthesis of a number of disilylated Yb(II), Eu(II), and Sm(II) complexes, 41 and more recently for a number of Ln(III) complexes. 31 NMR spectroscopic analysis of compound 4 shows it to be fairly similar to 2. Again the 29 Si NMR shift of the metallated Si atom of 4 (−179.9 ppm) is close to that of the starting material 3 (−185.7 ppm), 41 indicating a strong silanide character (Table 2), which is significantly more pronounced than in the corresponding magnesium compound ( Table 2). 6 A comparison of Tables 1 and 2 seems to suggest that the siloxane-containing 1,5-oligosilanylene ligand (Table 2) is of diminished silanide character as the chemical shifts are typically shifted down-field compared to the all-silicon 1,4treasilanylene ligand (Table 1). This is true for the listed metals: potassium, magnesium, calcium, and ytterbium. However, for yttrium, a reversed trend seems to occur. To understand this, it needs to be pointed out that while the chemical silanide shift is a good approximation for the silanide character, the nature of the ligand and the counterion are not the only variables here. In particular, solvent effects on the shift can be substantial. A strongly coordinating solvent can compete with the silanide ligand for the counter-ion and thus increase the silanide character. Therefore, the nature of the coordinating solvent (THF vs. DME) and also the solvent used for the NMR experiment contribute to the silanide shift.
For instance, Table 1 shows that the Yb complex with DME displays a stronger up-field shift than the related THF complex. The fact that the YCl 2 complex of the siloxane ligand (XI) (δ = −161.6 ppm, Table 2) displays stronger silanide character than 7 (δ = −154.7 ppm, Table 1) is in part caused by the fact that it was measured in DME, whereas 7 was measured in the less coordinating solvent THF-d 8 . 39 Single-crystal XRD analysis of 4 ( Figure 2) shows significantly elongated Ca−Si interactions of 3.1160 (10)  Finally, we were interested in converting the ferrocene-based oligosilanyl dianion 5 42 to the respective [3]-ferrocenophane calcium compound 6 (Scheme 3). The reaction proceeds again smoothly and 29 Si NMR analysis of 6 reiterates the trend previously observed for compounds 2 and 4. Again, the strong silanide character of 6 is reflected by a chemical shift (−116.7 ppm), close to that of the starting material 5 (−121.1 ppm) and shifted to a higher field than that of the corresponding   (Table 3).
Single-crystal XRD analysis of 6 ( Figure 3) shows a complex with two donating DME molecules and a hexacoordinate calcium atom similar to what was found for 2. As a consequence, the Ca−Si distance of 3.0765 (8)  Silyl Yttrium Compounds. Our recently reported reaction of the siloxane-containing oligosilanylene dianion 3 with YCl 3 to the respective disilylated dichloro yttrate-complex XI, 31 led us to reconsider the reaction of dianion 1 in a similar way. Indeed, the reaction of 1 with yttrium trichloride in DME proceeded smoothly, and we obtained dichloroyttratetrasilacyclopentane 7 as an ate-complex with a DME coordinating to the yttrium atom. 29 Si NMR spectroscopic analysis of 7 showed a doublet signal at −154.7 ppm for the metallated silicon atom with a 1 J Si−Y coupling constant of 56 Hz for the metalated silicon atom (Table 1). For the related product XI (see Chart 2), 31 the respective signal was detected with a chemical shift of −161.6 ppm and a 1 J Si−Y coupling constant of 38 Hz. Although these values are indicating a more ionic character of complex XI compared to 7, some reasons for this somewhat counterintuitive finding are discussed above.
The solid state structure of 7 (Figure 4), features Si−Y distances of 2.9589(9) and 2.9704(9) Å, about 0.1 Å shorter than those found for XI [3.064(2) and 3.057(1) Å]. 31 All other distances, such as Y−Cl and Y−O are very similar for both types of complexes.
It therefore seems valid to assume that the additional Y−O interaction, which distinguishes complex XI from complex 7, is responsible for the increased ionic character of the Si−Y bonds of the former.
Reaction of complex 7 with two equivalents of NaCp proceeds under substitution of the chlorides against cyclopentadienyl ligands to give the yttracenate complex 8 (Scheme 4). The 29 Si NMR spectrum of 8, with a signal at −152.8 ppm ( 1 J Si−Y = 57 Hz) for the metalated silicon, is fairly similar to that of 7 both with respect to chemical shift and 1 J Si−Y coupling constant ( Table 1), suggesting that the degree of covalency of the Si−Y bonds in both compounds is very similar. This is consistent with the fact that the Cp 2 Y ate complex XII, features only a small degree of interaction between the Y atom and the siloxane oxygen atom. 31 While the solid state structure of 8 ( Figure 5) features longer Si−Y [3.006(3) and 3.0106(13) Å] bonds than 7 [2.9589(9) and 2.9704(9) Å], these distances are substantially shortened compared to the 3.1315(9) and 3.1459(9) Å observed for complex XII. 31 Silyl Iron Compounds. As outlined in the introduction, Fe(II) complexes with two silyl groups are not completely uncommon. They are, however, paramagnetic and NMR characterization is not easily possible. Arata and Sunada showed that 1 H NMR spectra of [(Me 3 Si) 3 Si] 2 Fe(THF) 2 and [(Me 3 Si) 3 Si] 2 Fe(py) 2 (XIV) can be obtained but feature very broad and strongly shifted lines. 34 Our approach to cyclic disilylated Fe(II) complexes follows the same strategy as outlined above for the reactions with CaI 2 and YCl 3 . Reactions of oligosilanylene diides 1 and 3 with FeBr 2 thus proceeded to form compounds 9 and 10, respectively, as purple and ruby-red crystals (Scheme 5).
The 1 H NMR spectrum of 9 is similar to that of [(Me 3 Si) 3 Si] 2 Fe(THF) 2 34 with the OCH 2 THF signals shifted to ca. + 25 ppm. Unfortunately, the crystals of 9 were not suitable for single-crystal XRD analysis, but those of 10 proved to be acceptable and structure analysis was possible ( Figure 6).
As outlined above, we are aware of only three examples of structurally characterized bis(oligosilanylated) Fe(II) comp l e x e s . T h e s e a r e T i l l e y ' s a t e -c o m p l e x E t 4 N -{[(Me 3 Si) 3 Si] 2 FeCl} (XIII) 32 and Sunada's [(Me 3 Si) 3 Si] 2 Fe-(py) 2 and [(Me 3 Si) 3 Si] 2 Fe(THF) 2 (XIV) Chart 3). 34 These compounds feature Si−Fe bond distances of around 2.50 Å (Table 4) and the structural features of 10 match with those of  The conformation of 10 is somewhat unusual. It features a typical large Si−O−Si angle of almost 143°, which we have observed for several other 1-metalla-4-oxa-tetrasilacyclohexanes. This large angle renders the Si−O−Si unit almost as one ring side, thus allowing describing its conformational properties as those of a five-membered ring. While most of the 1metalla-4-oxa-tetrasilacyclohexanes studied by us so far tend to engage in a distorted envelope conformation with the metal unit as flap, 6,43 the structure of 10 is more accurately described as a half-chair conformer with the Si−Fe−Si unit and the oxygen atom in a plane and the SiMe 2 extending below and above the plane (Figure 6).

■ CONCLUSIONS
In our ongoing pursuit to develop the chemistry of heterocyclosilanes, we used a choice of oligosilanylene diides to prepare heterocyclic silanes with endocyclic Ca, Y, and Fe atoms. Calcacyclosilanes were obtained by reaction of oligosilanylene dianions with CaI 2 . 29 Si NMR spectroscopic analysis of the compounds indicates a retained strong silanide character, which is markedly more pronounced than in the analogous magnesacyclosilanes and almost resembles that of the potassium silanides used as starting materials. A degree of reactivity located between potassium and magnesium silanides would render silyl calcium compounds interesting silyl anionic reagents.
The reaction of a dipotassium 1,4-oligosilanylene diide with YCl 3 gave a five-membered yttracyclosilane as an ate-complex with two chlorides still attached to the yttrium atom. Treatment of this compound with two equivalents of NaCp led to a five-membered yttracyclosilane ate-complex containing an yttracene fragment.
NMR spectroscopic analysis of experiments, subjecting the obtained calcacyclosilanes 2 and 4 to reactions with YCl 3 showed the 29 Si signatures of 7 and XI, respectively. However, these reactions were less clean than those utilizing the potassium silanides 1 and 3, displaying a small amount of Table 3. 29 Si NMR Data of [3]Ferrocenophanes with 2-Metalla-1,3-disila Bridges
1 H (300 MHz), 13 C (75.4 MHz), and 29 Si (59.3 MHz) NMR spectra were recorded on a Varian INOVA 300 spectrometer. If not noted otherwise all samples were measured in C 6 D 6 . To compensate for the low isotopic abundance of 29 Si, the INEPT pulse sequence was used for the amplification of the signal. 47−49 Spectra are calibrated to the deuterium resonance of the solvent (C 6 D 6 ) 50 and referenced to tetramethysilane (TMS). 51 Elementary analyses were carried out using a Heraeus VARIO ELEMENTAR.
X-ray Structure Determination. For X-ray structure analyses, crystals were mounted onto the tip of glass fibers, and data collection was performed with a BRUKER-AXS SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (0.71073 Å). The data were reduced to F o 2 and corrected for absorption effects with SAINT 52 and SADABS, 53,54 respectively. The   58 and rendered using POV-Ray 3.6. 59

-C a l c a -2 , 2 , 5 , 5 -t e t r a k i s ( t r i m e t h y l s i l y l ) -tetramethylcyclopentasilane·(DME) 1.5 (2).
A solution of 2,2,5,5tetrakis(trimethylsilyl)decamethylhexasilane (101 mg, 0.17 mmol) and potassium tert-butoxide (39 mg, 0.35 mmol) in THF (2 mL) was stirred at rt for 19 h. Full conversion to disilanide 1 was confirmed by 29 Si NMR. Volatiles were removed under reduced pressure, the residue dissolved in DME (1.5 mL) and the resulting bright yellow solution was added dropwise within 2 min to a slurry of calcium diiodide (52 mg, 0.18 mmol) in DME (1 mL). The mixture was stirred for 2 h before the precipitated potassium iodide was removed by filtration. Colorless crystals of 2 (180 mg, 81%) were obtained from a toluene/DME solution. NMR (δ in ppm, d 8